1. Field of the Invention
The present invention relates to a method and apparatus for sensing fluid behaviour in a conduit, such as in a borehole. More particularly, but not exclusively, the present invention relates to measurement of multi-phase fluid flow within a borehole, and is of particular application to production logging and other in-hole flow measurement within well bores.
2. Description of Prior Art
Techniques are known for measuring the flow of fluids along pipelines, for example, utilizing sensors adjacent to, or incorporated in, the wall of the pipeline. Such sensors utilize one or another characteristic of the fluid in the pipeline to detect the movement of the fluid, and to produce output signals indicative of such movement. Such output signals may be processed as selected to provide information indicative of the fluid flow along the pipeline.
Such output information from two sensors may be cross-correlated to provide the flow information.
In the accompanying drawings, FIG. 1 is a schematic, fragmentary diagram of a prior art flowmeter including two sensors positioned for determining fluid flow along a pipeline which is shown in cross section; and
FIG. 2 is a graph of two suggested sensor signal traces, and a graph obtained by cross-correlation.
Referring to FIG. 1, the prior art flowmeter is shown generally at 10, proportioned for determining flow of fluid within a pipeline 12. The arrows at the ends of the segment of the pipeline 12 indicate the direction of fluid flow within the pipeline. The fluid may be multi-phase, as generally indicated. Two sensors 14 and 16 are mutually displaced along the pipeline 12 so that one sensor 14 is upstream of the other sensor 16. Each of the two sensors 14 and 16 is exposed to, and takes measurements within, a sampling volume within the pipe 12. As indicated schematically, the upstream sensor 14 samples within volume A, and the downstream sensor 16 samples within volume B. The sensors are chosen to measure, or sense, some induced or intrinsic physical property of the flowing mixture of material within the pipe 12. Each sensor produces an output signal indicative of the measured physical property of the flowing media. The output signals may be processed, or reduced, as selected to provide data signals including information indicative of the flow of material in the respective sampling volumes A and B. The two data signals may then be cross-correlated. In the flowmeter of FIG. 1, the output signal of the upstream sensor 14 is initially processed in circuitry 18, and the output signal of the downstream sensor 16 is initially processed in circuitry 20. Data signals from the processing circuitry 18 and 20 are directed to a processor 22 which cross-correlates the data signals.
FIG. 2 indicates, generally schematically, data signals obtained indirectly from the upstream and downstream sensors. The data signals in each case are characteristic of the mixture sensed at the time in the respective sample volume within the pipe. For purposes of illustration, a significant peak is shown in the upstream data signal U. A similar, corresponding peak occurs in the downstream signal D at a time t later. The time t is the time required for the volume of material which produced the peak in signal U, when within the sample volume A, to move downstream into the sample B to produce the peak in the downstream signal D.
It will be appreciated that, as the fluid moves along in the flow, the particles and the components of the fluid will retain their relative positions only momentarily. Consequently, a sensor "image" of the sample area encompassing the same, or essentially the same, portion of the fluid, will vary, depending on how much later the second sensor detects, or reads, the sample. The time lag is, of course, related to the longitudinal speed of the fluid flow as well as the longitudinal separation between two sensors whose output is being compared. Additionally, the nature of the flow, e.g. its degree of turbulence, affects the rate at which the configuration of the fluid within the volume changes as the volume of fluid moves downstream. The nature of the flow, in turn, is dependent on characteristics of the fluid as well as the pipeline, or flow container. Consequently, the signal images produced by two sensors, one downstream of the other, when detecting the passage of the same segment of fluid will not be identical; on the contrary, the signal images will differ depending on the variation in the configuration of the segment of the fluid flow as that segment is detected by each of the two sensors. Consequently, it may be difficult to distinguish a profile feature of one sensor signal, characteristic of the fluid configuration sampled, and identify that same signal profile in the output signal from the downstream sensor based on the latter's detection of the same segment of the fluid. Consequently, the time of passage of the fluid between the sensor positions may not be readily determined by simply comparing output plots of the two sensor signals.
One way of making the comparison is to use so-called cross-correlation to determine the time delay in fluid flow between two sensors which are displaced relative to each other. The cross-correlation technique includes taking samples of the two signals to be compared, and shifting one signal sample relative to the other on a time scale, multiplying the two signals together and taking the average of the result. The cross-correlation output will generally not show any pronounced structure assuming the output signals are not periodic. However, when the signal time shift matches the actual time delay, that is, the time required for the segment of the fluid to flow from one sensor to the other whereby the two signals being compared were obtained by the respective sensors detecting the same fluid segment, the correlation process effectively multiplies the signal by itself, thereby calculating the mean squared value of the original signal to produce a pronounced peak in the cross-correlogram. The time shift of one signal relative to the other, required to obtain this maximum correlation output, identifies the fluid flow time delay, that is, the time for a segment of fluid to flow the distance of separation of the two sensors. Since this distance may be known, a calculation of the flow speed is readily made.
Correlation flowmeters are known for use with pipelines whereby the flowmeter apparatus is exterior to the pipeline with, in some cases, the sensors being directly exposed to the flow by being placed in the pipeline wall, for example. We have now devised apparatus for sensing fluid behaviour in a conduit, which device can be used as a flowmeter within a conduit, or tubular member, along which fluid may flow, with the flowmeter contained within the conduit, at least in part. Further, in accordance with the invention, the apparatus can be used for obtaining flowmeter measurements within a pipeline without the need for flowmeter apparatus exterior to, or even within the wall of, the pipeline at the location at which measurements are taken.